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Randomized trial · measured the question itself

Trial in 923 Chicago adults: vitamin D doses did not clearly change COVID-19 risk

Meltzer and colleagues · Clinical nutrition (Edinburgh, Scotland) · 2026

Original title: Effect of moderate or high versus low dose daily vitamin D on COVID-19 incidence: A double-blind, parallel-group randomized clinical trial.

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What the study found

No difference

COVID-19 in 10.9% (50/461) on moderate or high dose vs 14.7% (68/462) on low dose (aHR:0.73; 95%-CI:0.51-1.06; P = 0.10). Moderate vs low: aHR:0.62 (0.39-0.99). High vs low: aHR:0.99 (0.53-1.86).

The study at a glance

Type of study
Randomized trial
Who or what was studied
923 people · Adults in Chicago who chose a moderate-vs-low or high-vs-low dose branch
What was tested
4000 or 10,000 IU of vitamin D a day vs 400 IU a day, for up to 9 months
What it measured
The question itself: colds, flu and other respiratory infections
Daily amount
10000 IU a day
How long
39 weeks
Trial registration
NCT04868903
Who has a stake
No stake declared
No competing interests declared; funded by an NIH institute.
Journal
Clinical nutrition (Edinburgh, Scotland), 2026
Source
PubMed record 42480239. This page is based on the abstract only.

How much this study counts

  • ✓Randomized: people were put into groups by chance, so the groups start out alike.
  • ✓Large: 923 people.
  • •Lasted 39 weeks.
  • ✓Measured the question itself: colds, flu and other respiratory infections.
  • ✓No competing interests declared.
  • •Summarized from the PubMed abstract only. The full paper may hold details the abstract leaves out.

✓ Strength– Limit• Note

These points come from fixed rules about the type of study, its size and length, what it measured, and who has a stake. Every study on Digestery gets the same rules.

Where this study sits

Each dot is one on-topic paper on this question. Steps near the top answer the question more directly.

↑ Stronger evidence

  1. Trials in people: colds, flu and chest infections

    10

    2 benefit, 8 no difference

    ↑ This study is the ringed dot

  2. Trials in people: other measures

    5

    1 benefit, 1 mixed, 2 no difference, 1 harm

  3. Trials in people: lab tests or safety

    1

    1 no difference

  4. Population studies

    1

    1 no difference

  5. Blood levels and genes

    4

    3 benefit, 1 no difference

↓ Weaker evidence

  • Points to benefit
  • Mixed
  • No difference
  • No direction
  • Points to harm

How it fits with the other research

The short answer to “Does vitamin D prevent colds and infections?”

Not clearly; any effect looks small.

Confidence: low. Based on 33 on-topic papers.

Read the full answer

Of the other 9 papers on the same step: 2 benefit, 7 no difference. This one found no difference. It agrees with most of them.

Other papers on the same step

See all 33 studies on this question

How this page was made

Label

First read of the abstract
Claude Sonnet
Independent second read
Claude Haiku
Differences settled
The two reads differed. The reviewer checked the abstract and kept the first read. Claude Opus 5.5.
Headline
Claude Sonnet, from the checked study record. A script checked its numbers and wording.
Numbers
Checked against the abstract by a script
“How much this study counts”
Fixed rules, the same for every study
Source check by a person
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